The Dounreay Fire Brigade was called immediately and the area was made safe.
The fire was quickly extinguished and the area confirmed to be safe within 30 minutes.
There was no harm to personnel.
The regulators and the NDA have been informed and a local investigation will take place.
Work in the sodium tank farm has been stopped.
PFR ceased operating in 1994 and is more than halfway through its decommissioning.
More than 1500 tonnes of sodium has been safely destroyed to date.
This incident occurred in the building where the tanks are used to store sodium.